In modern forestry, tree stands are in most cases thinned by means of e.g. "one-grip" harvesters. A frequent problem in such thinning is caused by the thicket or shrub layer which is often found adjacent the trees which are to be thinned and cut. Such thicket normally grows around the lower part of the tree to be cut, thus preventing a clear view by the person who operates the harvester from a driving-cab located a considerable distance from the tree. Besides blocking the operator's view, the thicket also causes problems by easily finding its way between the the trunk and the saw chain mounted on the guide bar of the harvester, which serves as a cutting tool. As a result, the chain may slip or jam and, consequently, lose its capacity of effectively cutting the trunk. To eliminate the problems caused by thicket surrounding the tree, in practice there is usually a special operator who manually cleans the tree stand from the shrub layer by means of a conventional bush saw, before the actual thinning is carried out. However, this work is costly and involves a number of drawbacks. One drawback is that the cleaning operator does not know in advance which direction the harvester will be moving towards the tree to be thinned. Thus the operator must remove all the thicket in a 360.degree. sector around the tree. Removing substantially all the thicket in this manner while leaving completely bare surfaces around the tree is contrary to recent environmental protection practices which recommend a certain amount of shrub layer in a growing forest, to create a favourable environment for forest animals. Such clear-cleaning involves extensive work which takes much time and thus is expensive. In practice, the cost for manual thicket-cleaning alone is estimated at 15-20% of the total thinning cost. A further problem relates to the technical planning, particularly for large forest owners with a large number of scattered stands. Thus, it is extremely difficult to plan the cleaning operator's work to coordinate efficiently with the thinning operation.